In the field of magnetic anti-counterfeiting of banknotes, recording head technology, magneto-resistance (MR) head technology and giant magneto-resistance (GMR) effect technology are mainly used in the current market; in the field of anti-counterfeiting of bills, recording head technology is mainly adopted.
Recording head technology: This recording head uses permalloy or other soft magnetic materials to make a ring structure, which has a gap and is wrapped with a coil. When the air gap of the ring structure quickly passes the surface of a magnetic pattern, induced current will be generated inside the coil based on Faraday's law of electromagnetic induction. By detecting the changes of induced current, the changes of the magnetic field on the surface of the magnetic pattern are known. Main disadvantages of this technology are: 1. It is more suitable to detecting magnetic field on the surface of a hard magnetic material. When it detects soft magnetic material, a background field needs to be added, but this background field will affect the sensitivity of the magnetic head to a great extent; 2. When multichannel heads are made, it is very difficult to assure uniformity of sensitivity in all channels, resulting in a low yield rate and high mass production cost; 3. The recording head has very low immunity from interference of external magnetic fields and requires necessary magnetic shielding against various kinds of interference sources in a complex working environment. This raises the cost of subsequent application products on the one hand and the design difficulty of subsequent application products on the other hand; 4. The output amplitude of signals is relevant with the speed of the recording head relative to the detected magnetic pattern, which adding difficulty to quantitative analysis of signals; 5. Under the condition that some magnetic fields exist indeed, but they don't have a large spatial difference, the actual output of the magnetic head is very small and the magnetic fields may not be detected.
Magneto-resistive head technology: This magnetic head uses InSb or other magneto-resistive materials. Two magneto-resistors are used to constitute a Wheatstone half-bridge. They have spatial location difference in a plane. By detecting the difference between the magnetic field strength on the two magneto resistors, the magnetic field gradient on the surface of the magnetic pattern is detected. Disadvantages of this technology: 1. In this technology, a bias magnetic field must be added in the vertical direction of the magneto-resistive elements so that magneto-resistive elements can work. Meanwhile, as the soft magnetic pattern needs to be magnetized perpendicular to the surface, the needed magnetic field is very large in general. In order to generate this magnetic field, permanent magnets with strong magnetism are needed to reduce magnetism attenuation, but the inevitability of magnetism attenuation results in possible changes of MR sensitivity. In other words, with the increase of time, the sensitivity of the magnetic head may be changed. 2. The sensitivity of MR elements is low, in the range of 0.2 mV/V/Gs-1 mV/V/G in general. This requires a large amplification factor of the subsequent signal processing circuit when a weak magnetic field is measured (in general, the magnetic field strength of soft magnetic pattern on banknotes is low). 3. MR elements are noisy. During detection of weak magnetic field signals, sophisticated frequency-locking and amplification technologies are needed. As a result, the detectable frequency of the magnetic field signals is limited to some extent. 4. As it adopts a half-bridge structure, it has limited immunity from interference of power supply fluctuations and external coupling signals and requires necessary magnetic shielding against various kinds of interference sources in a complex working environment. This raises the cost of subsequent application products on the one hand and the design difficulty of subsequent application products on the other hand. 5. When the magnetic head is multi-channel structure, it is very difficult to assure the uniformity of sensitivity of all channels, resulting in a low yield rate and high mass production cost. 6. As the design adopts a gradiometer, only the spatial gradient of the vertical component of the magnetic field can be identified and the actual distribution of the magnetic field cannot be measured. Under the condition that some magnetic fields exist indeed, but they don't have a large spatial difference, the actual output of the magnetic head is very small and the magnetic fields may not be detected.
GMR technology: This technology may adopt a thin-film process. The sensing axis is in the thin film's plane. The magnetic head based on this technology is realized mainly by two methods: In one method, two GMR resistors are prepared on the substrate. The sensing axises of the two GMR resistors may be consistent or opposite. The two GMR resistors are connected into a form of Wheatstone half-bridge. The plane where the two placed GMR resistors are parallel with the plane where the magnetic pattern to be detected is located. Meanwhile, the relative positions of the two placed GMR resistors may be parallel with or perpendicular to the moving direction of the magnetic pattern. These two methods are both used to detect the gradient value of the in-plane component of magnetic field on the surface of the magnetic pattern in the arrangement direction of the two in-plane GMR resistors. Advantages of this technology are: As a thin-film process is adopted, the uniformity of the products can be easily assured. Disadvantages of this technology are: 1. As gradient detection method is adopted, the actual distribution of the magnetic field cannot be directly measured. Under the condition that some magnetic fields exist indeed, but they don't have a large spatial difference, the actual output of the magnetic head is very small and the magnetic fields may not be detected. 2. As it adopts a half-bridge structure, it has limited immunity from interference of power supply fluctuations and external coupling signals and requires necessary magnetic shielding against various kinds of interference sources in a complex working environment. This raises the cost of subsequent application products on the one hand and the design difficulty of subsequent application products on the other hand.